C
Additional Solar System Exploration Mission Concepts

RPS-ENHANCED OR RPS-ENABLED MISSIONS ENVISIONED BY 2020

Additional Long-Lived Lander Concepts

In addition to the Venus Long-Lived Lander described in Chapter 4, other examples of long-lived landers enhanced or enabled by radioisotope power systems (RPSs) are, in heliocentric order, as follows:

  • Mercury Polar In Situ Explorer. Radar reflectivity measurements of polar craters on Mercury indicate that a reflective material, believed to be ice, exists locally. Because the craters of interest lie near the poles and may be in permanent shadow, RPSs will be needed to enable surface and shallow subsurface sampling, to distinguish between surface volatile deposits and bedrock or regolith.

  • Mars Deep Driller. The Mars Exploration Rovers have revealed clear evidence for near-surface (several to tens of meters) stratigraphy on Mars, which contains fundamental information about the history of Mars and the history of water within those regions of Mars that were once wet. Drilling can potentially reach depths of up to 1 km, sampling layers that are largely unmodified by near-surface chemical processes, and may reach layers that are not exposed for view anywhere on the surface. The speed and depth of drilling need to be matched to available power sources. Slow, shallow drilling may be within the capabilities of an RPS, but deeper, faster drilling may require power from a small nuclear reactor. An RPS-powered mission of this type has been studied for launch sometime in the next decade.1

  • Mars Polar Profiler. A desire to understand long-term climatic variations would lead to analysis of the depth structure of a polar cap, with in situ down-hole instrumentation perhaps augmented with retrieval of cored material to the surface for detailed analysis.

  • Io In Situ Explorer. Io is probably the most volcanically and tectonically active body in the solar system. Io’s atmosphere is uniquely affected by ubiquitous and time-variable volcanism, which adds to the atmospheric inventory through plumes and affects the surface temperature and composition. Monitoring of Io’s volcanism over a significant period (at least several months) would allow study at a range of resolutions and timescales of the volcano-tectonic processes on this highly active body. The extreme temporal variability of Io requires a high data rate to adequately monitor surface changes. Development of radiation-hardened electronics and instruments would be required to allow a surface package to endure for a sufficient length of time to monitor activity in Jupiter’s harsh radiation environment.



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Priorities in Space Science Enabled by Nuclear Power and Propulsion C Additional Solar System Exploration Mission Concepts RPS-ENHANCED OR RPS-ENABLED MISSIONS ENVISIONED BY 2020 Additional Long-Lived Lander Concepts In addition to the Venus Long-Lived Lander described in Chapter 4, other examples of long-lived landers enhanced or enabled by radioisotope power systems (RPSs) are, in heliocentric order, as follows: Mercury Polar In Situ Explorer. Radar reflectivity measurements of polar craters on Mercury indicate that a reflective material, believed to be ice, exists locally. Because the craters of interest lie near the poles and may be in permanent shadow, RPSs will be needed to enable surface and shallow subsurface sampling, to distinguish between surface volatile deposits and bedrock or regolith. Mars Deep Driller. The Mars Exploration Rovers have revealed clear evidence for near-surface (several to tens of meters) stratigraphy on Mars, which contains fundamental information about the history of Mars and the history of water within those regions of Mars that were once wet. Drilling can potentially reach depths of up to 1 km, sampling layers that are largely unmodified by near-surface chemical processes, and may reach layers that are not exposed for view anywhere on the surface. The speed and depth of drilling need to be matched to available power sources. Slow, shallow drilling may be within the capabilities of an RPS, but deeper, faster drilling may require power from a small nuclear reactor. An RPS-powered mission of this type has been studied for launch sometime in the next decade.1 Mars Polar Profiler. A desire to understand long-term climatic variations would lead to analysis of the depth structure of a polar cap, with in situ down-hole instrumentation perhaps augmented with retrieval of cored material to the surface for detailed analysis. Io In Situ Explorer. Io is probably the most volcanically and tectonically active body in the solar system. Io’s atmosphere is uniquely affected by ubiquitous and time-variable volcanism, which adds to the atmospheric inventory through plumes and affects the surface temperature and composition. Monitoring of Io’s volcanism over a significant period (at least several months) would allow study at a range of resolutions and timescales of the volcano-tectonic processes on this highly active body. The extreme temporal variability of Io requires a high data rate to adequately monitor surface changes. Development of radiation-hardened electronics and instruments would be required to allow a surface package to endure for a sufficient length of time to monitor activity in Jupiter’s harsh radiation environment.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Europa Astrobiology Lander. The harsh radiation from the jovian magnetosphere breaks all chemical bonds in the top tens of centimeters of Europa’s surface, making it desirable that an astrobiology lander be capable of deriving material from a depth of at least 50 cm to analyze it for signatures of biological precursors or activity. Icy Satellite Deep Driller. A study of ice cores from the tectonically active icy satellites will shed light both on the history and the chemical composition of any possible subsurface oceans. The deep driller should be capable of drilling down to, and analyzing samples (cores or well logs) taken from, depths of tens of meters. The challenge for such a mission is the fabrication of a remotely operated deep drill, which would require substantial operating power (several hundred watts). The drill could be installed on a mobile platform (like a rover) to provide multiple measurements from different geological regions. Deep/fast drilling capability may require power from a nuclear reactor. Comet Nucleus and KBO Surface laboratories. Primitive bodies are diverse, ranging from asteroids in the main-belt and near-Earth space, to comets passing through the inner solar system, to Kuiper Belt objects. Before the return of a cryogenic sample from a comet nucleus back to Earth, much information could be collected about the nature of cometary ices and volatiles through in situ sampling of the ices. The low surface gravity of these bodies and the unknown nature of the surface materials would make developing such a laboratory a challenge. Additional Rover Concepts Rover concepts enhanced or enabled by RPS technology are, in heliocentric order, as follows: Venus Mobile Laboratory. Such a mission would couple the challenge of building a rover that could investigate a larger area than allowed by a simple lander with the challenge of operating any equipment in such a hostile environment. Lunar Polar Rover/Driller. Permanently shadowed craters at the lunar poles are believed to contain concentrations of hydrogen or its compounds, which may provide a resource for human use. To assess the scientific and resource potential of these deposits will require three-dimensional investigations within permanently shadowed craters. Long-lived rover and drilling missions require moderate amounts of power (hundreds of watts) for operation, which would be difficult to obtain from solar cells alone at the basin’s location near the Moon’s south pole. RPSs, on the other hand, could provide long-term thermal and electric power and, thus, enable a rover equipped with a 1- to 2-m coring device to range over distances of ~10 km and operate continuously through the long, cold lunar night. Mars Advanced Science Laboratory. It is possible to envisage more elaborate versions of NASA’s planned Mars Science Laboratory equipped with, for example, more capable analytic instruments and the ability to drill beneath Mars’s thin, hostile near-surface layer. These capabilities, plus extended range and endurance, are significantly enhanced by the availability of power at a level of hundreds of watts or greater from RPSs. A somewhat similar concept, the Astrobiology Field Laboratory, has been studied for possible launch to Mars sometime in the next decade.2 Titan Surface Laboratory. The heliocentric distance of Titan, coupled with its dense atmosphere, makes the use of an RPS power source critical for any exploration on the surface. The images returned by the Huygens probe and Cassini’s radar system indicate that significant parts of Titan’s surface have the kind of muted relief that is ideally suited for long-range rover operations. A Mars Exploration Rover (MER)- or Mars Science Laboratory (MSL)-class rover, powered by an RPS, might be a less complex and risky approach to the exploration of Titan than the aerobot concepts discussed below and elsewhere in this report. Additional Global Network Concepts In addition to the Long-Lived Mars Network described in Chapter 4, other examples of network missions enhanced or enabled by RPSs are, in heliocentric order, as follows: Mercury and Lunar Long-Lived networks. These missions are similar in concept to the networks that could

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Priorities in Space Science Enabled by Nuclear Power and Propulsion be deployed on Mars or Venus, except that the payload would emphasize measurement of seismic activity and heat-flow information. The heat-flow probes should be implanted under the surface using a drill or other means. Icy Satellite Long-Lived Networks. Seismic and magnetic sounding and measurements of geothermal heat flow are required to understand the internal structures and the states of the icy moons. The icy satellites of the solar system are some of the coldest places visited by spacecraft, and Galilean satellite surfaces are also exposed to the harsh radiation emanating from the jovian magnetosphere. Therefore, surface geophysical observatories require protection from these cold and harsh radiation environments. To obtain useful information about the interior, measurements from several landing sites over a period of several weeks to months would be required. The heating and long-term power requirements can be met through the use of RPS and radioisotope heater unit (RHU) technologies. Additional Sample-Return Concepts In addition to the Cryogenic Comet Sample Return, other examples of sample-return missions enhanced or enabled by an RPS are, in heliocentric order, as follows: Mercury Sample Return. Returning a sample from the surface would allow Mercury to be placed within the context of solar system chemistry and would provide clues to formation processes in the inner solar system. Venus Selected Sample Return. Sample return allows detailed geochemical analyses (e.g., of rare-earth elements and various isotopic systems) that constrain models for crustal and mantle evolution. Since Venus is essentially Earth’s twin but has clearly undergone a very different history, studying its petrology, mineralogy, and trapped volatiles is important to understanding both Venus and Earth. Mars Cryogenic Sample Return. This mission would extract one or more cores of material (>10 m) from a polar cap or from subsurface ice deposits, and would maintain them in a sealed, refrigerated state for return to Earth. The preservation of samples of volatile materials at cryogenic temperatures is significantly enhanced by the availability of RPSs. Additional Aerobot Concepts Possible aerobot concepts enhanced or enabled by RPSs are, in heliocentric order, as follows: Venus Aviator. An aerial platform with maneuvering capability within the atmosphere of Venus would uniquely allow measurement of the three-dimensional composition and dynamics of the atmosphere. Measurements of expected volcanic gases by this vehicle would allow pinpointing and monitoring of volcanic emissions, if present. Flights at altitudes of less than 1 km would allow for high-resolution infrared mapping of the surface. Titan Aerobot Explorer. This mission would involve an RPS-powered lighter-than-air vehicle navigating in Titan’s atmosphere.3 Investigations would include high-resolution imaging of the surface from a variety of altitudes, possible subsurface sounding, measurement of weather phenomena, and, ideally, some analysis of surface material, either remotely or by a tethered sample collector. NEP-ENHANCED OR NEP-ENABLED MISSIONS ENVISIONED BY 2020 In addition to the Jupiter Icy Moons Orbiter (JIMO), the Titan Express/Interstellar Pioneer, and the Neptune-Triton System Explorer discussed in Chapter 6, other examples of missions enabled by NEP-propulsion capabilities are, in heliocentric order, as follows: Saturn System Multiple Rendezvous. This concept is for a JIMO-like mission performing a tour of the saturnian system, including orbiting several of the saturnian satellites. If the NEP system’s thrust is sufficient, “hovering” above the ring-plane may be possible. Delivery of (and provision of communications support to) sub-

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Priorities in Space Science Enabled by Nuclear Power and Propulsion spacecraft such as landers, Saturn probes, and so on may also be a feature. It is important to note that the saturnian radiation environment is less severe than Jupiter’s; thus the lifetime limitation for JIMO does not apply at Saturn. Transit time, spacecraft cleanliness, and maneuverability are significant concerns. NASA has already completed a detailed study of such a mission.4 Main-Belt, Trojan Asteroid, and Centaur Multiple Rendezvous. This mission concept could employ the versatile capabilities of nuclear power and propulsion to explore the diversity of main-belt asteroids and trace the compositional gradient of solar system small bodies to the jovian Trojan asteroids and beyond to the Centaurs. Using high-resolution spectral and spatial imagery, radio science, and instrumentation to determine surface composition and subsurface structure, this mission would open an entirely new window on understanding of the nature and origins of primitive bodies. By exploring the region from the asteroid belt to the Centaurs, this mission would investigate compositions ranging from those similar to the early Earth, to primitive material from the jovian accretion region of the nebula, to objects that might be dynamically evolved from the Kuiper Belt. NEP-ENHANCED OR NEP-ENABLED MISSIONS ENVISIONED AFTER 2020 Missions Deferred for Scientific Reasons Until After 2020 Although certain missions are clearly enabled by nuclear power and/or propulsion systems, scientific arguments can be made for delaying their launch until after 2020. Examples of such missions include the following: Titan Surface Sample Return. This mission is envisioned to include acquisition of Titan surface materials (including organics and ices) and their return to Earth in cryogenic condition. Titan ascent/descent may be performed by non-nuclear means, but sample return would require an advanced propulsion system. An in situ Titan exploration would define the goals of this mission and would be a likely precursor to a sample-return mission, hence providing the rationale for likely consideration after 2020. Uranus System Explorer. The ice-giant Uranus is unique in two key respects. First, all of the solar system’s giant planets except Uranus have measurable amounts of heat emerging from their interiors. Second, Uranus is the only giant planet that spins on its side—i.e., at an obliquity of 98°. Whether this unique feature has anything to do with the low-heat flux is not known, but clearly this radical obliquity defines Uranus as an extremum for studies of suites of giant-planet models. The uranian system also has an unusual magnetic field, a ring system characterized by dark, narrow, and in some cases eccentric rings, plus five major icy satellites as well as a plethora of smaller moons. A Uranus-system mission should be comprehensive and relatively long term, and its objectives should span the planet, rings, magnetosphere, and satellites. These two goals would be strongly enabled by nuclear technologies, specifically power for the breadth and diversity of instrumentation, and propulsion for crafting a suitable system tour; however, reasonable transit times must be obtained. Geometric considerations suggest flight of this mission after 2020. Current observations of Uranus suggest that its atmosphere undergoes significant seasonal changes as the planet approaches equinox. Voyager 2 flew past Uranus at almost precisely its southern summer solstice (pole pointing at the Sun and Earth). A Uranus System Explorer should be targeted to arrive at the planet near equinox, the more interesting season, i.e., 2007 + 42 years, or 2049. A 10-year flight puts launch in the late 2030s. Multiple Kuiper Belt Object Rendezvous. This mission concept envisages a spacecraft capable of long-term orbital operations among the most remote and most primitive regions of the solar system. Through exploration of the Kuiper Belt, fundamental new insights are expected in planetary formation and accretion processes. Using the propulsion maneuverability and power of the nuclear systems, a mission trajectory could target multiple Kuiper Belt objects selected for their diversity in size, composition, and single or binary configuration. Apart from addressing basic questions concerning the objects’ origin, composition, and morphology, detailed science investigations will be formulated that incorporate results from the first reconnaissance of Pluto and the Kuiper Belt by the New Horizons mission. Incorporating these mission results into the scientific planning and execution of a nuclear-enabled Multiple Kuiper Belt Object Rendezvous mission provides the rationale for launch in 2020 or beyond.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Missions Deferred for Technical Reasons Until After 2020 Examples of post-2020 missions enabled by Prometheus-derived propulsion include, in heliocentric order, the following: Icy Moons Subsurface Sample Return. Large icy, airless moons have surfaces that are greatly altered by external processes (e.g., bombardment by magnetospheric particle and micrometeorites, and ultraviolet chemistry and textural alteration). At a depth of centimeters to meters, the ice is less altered by these processes and contains information on the evolution of the body and possibly the conditions of origin. The specific goals vary from moon to moon. In the special case of Europa, there is likely to be a particularly high science return if the near-surface ice contains little modified material delivered from a subsurface ocean or melting event. This mission has very high technology demands because of the need to provide energy on the surface, as well as the need to deliver material out of Jupiter’s gravity well and cryogenically back to Earth’s surface. Meeting these demands would draw on nuclear propulsion and RPS capabilities of the type being developed by Project Prometheus. Main-Belt, Trojan Asteroid, and Centaur Multi-Sample Return. Performing the first detailed laboratory studies of the solar system’s compositional gradient is the primary science goal of this mission concept. With nuclear power and propulsion, sampling a broad range of solar system bodies and returning those samples to Earth would become a newly enabled capability. Key to this study is characterization of the volatile inventory of the solar system, with the goal to understand the source of the life-enabling volatiles on Earth. REFERENCES 1. See, for example, S. Miller, J. Essmiller, and D. Beaty, “Mars Deep Drill—Explore Active Hydrothermal Habitats,” Jet Propulsion Laboratory, Pasadena, Calif., 2004. Available online at <http://mepag.jpl.nasa.gov/Advanced_Mission_Studies/index.html>, last accessed February 2, 2006. 2. See, for example, R. Diehl, “Astrobiology Field Laboratory—2013 Biosignature Detection,” Jet Propulsion Laboratory, Pasadena, Calif., 2004. Available online at <http://mepag.jpl.nasa.gov/Advanced_Mission_Studies/index.html>, last accessed February 2, 2006. 3. J.L. Hall, V.V. Kerzhanovich, A.H. Yavrouian, J.A. Jones, C.V. White, B.A. Dudik, G.A. Plett, J. Mennella, and A. Elfes, “An Aerobot for Global In Situ Exploration of Titan,” Advances in Space Research, in press. 4. Team Prometheus, NASA/DOE, Saturn/Titan—JIMO Follow-On Mission Study, Final Report, NASA, Washington, D.C., July 9, 2004.